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        // 1-声明变量和赋值
        //  变量var ---es5
        //  变量let ===es6

        //  先声明后赋值
        let a;
        a=1;
        console.log(a);

        // 同时申明和赋值
        let b = 2
        console.log(b);

        let x,y,z=100;
        // let x1=y=z=100;

        console.log(x);
        console.log(y);
        console.log(z);


        // 不申明直接赋值   xxxxxxxxxx不推荐使用
        x = 88;
        console.log(x);




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        alert(null==undefined)///true

    // Number
        // 8进制==0
        // 16进制==0x
        // NaN


        document.write("nan:"+typeof(NaN))
        document.write("nan:"+typeof(nan))

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